1. Field of the Invention
The present invention is generally concerned with fluid couplings.
Generally speaking, fluid couplings comprise two members namely an impeller wheel forming a pump and an output wheel forming a turbine. Disposed face-to-face in a casing, these members define between them a working circuit containing fluid.
The present invention is more particularly directed to the case where the working circuit contains a fixed quantity of fluid at all times.
2. Description of the Prior Art
Fluid couplings of this type are used among other things to drive a load, being inserted between the load and the motor, and their function is firstly to run up to load progressively in order to protect the motor and secondly to introduce advantageous flexibility into the operation of the system.
In practice the drive torque available at the output shaft depends on the fluid capacity and the speed.
The fluid capacity in turn depends in particular on the power to be transmitted under steady state conditions.
Given the particular fluid capacity, the drive torque on starting up can be too high to achieve the required progressive operation, especially if the load to be driven has very high inertia, as can be the case with some conveyor belts, for example.
If nothing is done to prevent it, this starting torque is usually in a ratio in the order of 1.7 to 1.8 to the normal resisting torque under steady state conditions.
To alleviate this difficulty and achieve a more progressive start it is usual practice to provide a bypass chamber coupled to the working circuit and forming an annulus around the axis of the system. This is adapted to limit the quantity of fluid actually present in the working circuit on starting up and therefore the fluid capacity at this time, by temporarily diverting a significant proportion of the fluid.
In practice the communication between the working circuit and the bypass chamber comprises at least one distinct offtake channel and at least one return channel with at least one calibrated nozzle means on the return channel to achieve the required progressive operation.
These arrangements are usually satisfactory.
They make it possible to reduce the ratio between the drive torque and the normal resisting torque under steady state conditions to a value in the order of 1.5 to 1.7 (or even 1.3 to 1.5 if the bypass chamber is sufficiently long and the calibrated nozzle means of the return channel are appropriately specified).
They may still be unsatisfactory, however, in some cases at least, when it is desirable for the ratio to be reduced to a value in the order of 1.2, for example.
To overcome this problem it has previously been proposed to add to the first bypass chamber or main bypass chamber a second bypass chamber referred to hereinafter for convenience only as the auxiliary bypass chamber.
This is the case, for example, in the German patent application No 35 22 174.
However, in this document the offtake channel connecting the working circuit to the auxiliary bypass chamber uses the gap between the outside peripheries of the impeller wheel and the output wheel.
A first consequence of this is that the offtake of fluid is due to centrifugal force which requires that the rotation speed is sufficiently high and therefore involves some time-delay.
Another consequence is that the auxiliary bypass chamber must necessarily extend at least in part radially beyond the outer peripheral diameter of the working circuit, to the detriment of the overall diameter of the system and its inertia.
It is known to provide holes in the output wheel to balance the pressure on its opposite sides.
A general object of the present invention is an arrangement which further uses these holes to provide a very simple way to avoid the above problems.